Synchronous rectifiers, based on the use of power metal-oxide-semiconductor field-effect transistors (MOSFETs) to replace diodes for reducing conduction losses, have been used in low-voltage and high-current applications. Synchronous rectifier techniques are primarily applied to various versions of DC-DC converters such as buck converters, flyback converters, boost-buck converters, half-bridge converters, and LCC resonant converters. To reduce the cost of the gate drive circuits, self-driven techniques have been an active research topic in synchronous rectifiers, although a gate control integrated circuit for driving synchronous rectifiers is also commercially available. Other research aspects include the use of soft-switching techniques. Besides DC-DC converters, synchronous rectification techniques have been applied to three-phase full-bridge AC-DC converter based on a three-phase fully-controlled bridge and even to a five-level converter. While the self-driven technique uses the changing voltage polarity of the coupled windings to control the switching of the power MOSFETs, other techniques tend to use control integrated circuits to provide the gating signals. Another conventional approach replaces a general-purpose diode bridge with synchronous rectifier for low power and low voltage (e.g., 3V to 5V) applications in which the synchronous rectification technique is applied to a centre-tap rectifier topology. However, in such approach, a customized charge pump circuit is needed in order to provide a suitable DC power supply for the gate drive. Also, as this approach aims at low-voltage applications, it is not suitable for mains voltage operations.
A three-phase synchronous rectifier that can operate at mains frequency also has been developed and reported in U.S. Pat. No. 6,765,425. It is based on the detection of the phase-phase voltage, output voltage and timing circuits. Sophisticated logic and timing circuits are needed to provide the gating signals if the AC source has significant source inductance. However, the gating signals for synchronous rectifiers based on phase-phase and output voltages detection is not adequate because the diodes of a traditional bridge rectifier only turn off naturally after their current reverse-recovery processes. In U.S. Pat. No. 6,765,425, the three-phase synchronous rectifier circuit replaces the six diodes with power MOSFETs and uses “voltage-controlled gate drive circuits” and the appropriate logic circuits to control the switching of the six MOSFETs. Voltage control here refers to the detection of the output voltage and the AC input voltage or voltages which could be phase voltages or line voltages. To cope with different types of load, that synchronous rectifier circuit can be initially inactivated in order to allow the body-diodes of the 6 MOSFETs to conduct like a normal 3-phase diode rectifier. Using timing circuits, the conduction periods of the MOSFETs' bode diodes are then registered. Such conduction time information is then used to control the conduction time of the MOSFETS. However, this approach has at least three major limitations. First, with the use of voltage detection only, the logic circuits have to be tailor-made to cater for a particular application. Second, since a diode will turn off only when its current is reversed and has gone through the reverse-recovery process using voltage detection cannot guarantee equivalent diode bridge rectification functions under all types of loads and circumstances. Third, a DC power supply derived from the AC voltage supply is desired to power the control electronics. While a DC power supply for the control circuitry can be derived from the input AC voltage source with the aid of isolation transformers, transformers cannot be easily integrated into the same package with the MOSFETs in a compact way, and as a result, the control circuitry with the transformers will take up significant space and cannot be built in the same package with the MOSFETs, and as further result, will not be able to form a replacement block for a diode rectifier.
It is desirable to be able to provide a self-driven AC-DC synchronous rectification technique that can be used to develop an AC-DC synchronous rectifier that can behave like a diode bridge and be used in high-voltage power applications, but with significantly reduced conduction losses and without requiring control integrated circuits. It is also desirable to be able to use such an AC-DC synchronous rectifier in single-phase and multi-phase systems.